This invention relates to a low-cost pressure sensor assembly incorporating a stainless steel sensor element for attachment to a pneumatic valve.
Stainless steel pressure sensor technology has been successfully utilized in automotive applications involving high pressure fluids such as engine oil, brake fluid, and so on. The sensor element is usually formed as a shallow cylinder, closed at one end to define a diaphragm, and is welded to a pressure port adapted for attachment to a pressure vessel by a threaded fitting or the like such that the fluid contained by the vessel contacts the inboard side of the sensor diaphragm. Piezo-resistive elements formed on the outboard side of the diaphragm exhibit an electrical resistance variation corresponding to deflection of the diaphragm by the fluid pressure, and signal-conditioning circuitry responsive to the sensor element resistance develops an electrical signal representative of the fluid pressure. To permit welding of the stainless steel sensor element to the pressure port, the pressure port is usually also formed of stainless steel material. However, since weldable stainless steel material is relatively expensive and difficult to machine, it has been suggested to utilize a multi-piece assembly that minimizes the stainless steel content. See, for example, the U.S. Pat. Nos. 6,050,147, issued on Apr. 18, 2000, and 5,939,637, issued on Aug. 17, 1999, both of which are assigned to the assignee of the present invention. In the U.S. Pat. No. 6,050,147, the sensor element is welded to a stainless steel pressure port having a central axial bore, and a low-cost metal casing is attached to the stainless steel pressure port by means of an interlocking press-fit. In the U.S. Pat. No. 5,939,637, the sensor element is welded to a stainless steel insert having a central axial bore, where the insert is captured in a low-cost metal pressure port, and an O-ring is compressed between the insert and pressure port to provide sealing therebetween.
Despite being cost-effective, the above-referenced pressure sensor assemblies are not directly adaptable for usage in connection with a pneumatic valve having a spring-biased valve plunger. In such an application, a central stem or boss is required to depress the plunger as the pressure sensor assembly is attached to the valve, and such requirement is incompatible with a central axial bore. Accordingly, what is needed is a low-cost stainless steel pressure sensor assembly that is adapted to be attached to a pneumatic valve having a spring-biased plunger.
The present invention is directed to an improved low-cost pressure sensor assembly in which a stainless steel sensor element is welded to an outboard face of a stainless steel insert captured in a low-cost metal pressure port adapted for attachment to a pneumatic valve. The pressure port includes outboard and inboard cavities separated by neck portion. The stainless steel insert is captured in the outboard cavity, while the inboard cavity is adapted for attachment to the pneumatic valve. The stainless steel insert includes a base portion that seats against an O-ring in a floor of the outboard cavity, and a stem portion within the O-ring that extends axially through the neck portion of the pressure port and into its inboard cavity. The insert has a first bore that extends axially from its outboard face into, but not through, the stem portion, and a second bore that intersects the first bore and opens into the inboard cavity of the pressure port. When the pressure port is attached to the pneumatic valve, the stem portion of the insert depresses a spring-biased plunger that opens the valve, and fluid (gas or liquid) escaping through the open valve is placed in direct communication with the sensor element via the first and second bores of the stainless steel insert.